Process for preparing of creep resistant pressure-sensitive resins

ABSTRACT

A process comprising solution interpolymerizing a pressuresensitive resin containing an acid-epoxy monomer system, formulating the interpolymer solution with the 1,3bis(dimethylamino)-2-hydroxypropane and coating the solution on a substrate to obtain a film of pressure-sensitive resin thereon. The pressure-sensitive film possesses cohesive strength and exhibits low release values from silicone coated release paper.

United States Patent [191 Gardner et al.

[ 1 Sept. 2, 1975 PROCESS FOR PREPARING OF CREEP RESISTANTPRESSURE-SENSITIVE RESINS [75] Inventors: Donald M. Gardner,Springfield;

Lawrence W. McKenna, Wilbraham, both of Mass.

[73] Assignee: Monsanto Company, St. Louis, Mo.

[22] Filed: Feb. 20, 1973 [21] Appl. No.: 333,915

[52] U.S. Cl. 260/785 T; 117/76 A; 117/76 P; 117/122 P; 117/123 D;260/785 R;

[51] Int. Cl. C08F 8/30; C08F 20/06; C09] 7/04 [58] Field of Search260/8072, 80.8, 80.81, 260/785 R, 78.5 T; 117/122 P, 76 A, 76 P [56]References Cited UNITED STATES PATENTS 2,723,195 11/1955 Blake 92/33,563,953 2/1971 Lehmann et al 260/63 3,694,418 9/1972 FOck et a1260/785 R 3,697,618 10/1972 Grunewalder et a1. 260/785 R PrimaryExaminer-Joseph L. Schofer Assistant Examiner.lohn Kight Attorney,Agent, or' FirmR. B. Blance; E. P. Grattan; .1. C. Logomasini [5 7ABSTRACT 5 Claims, No Drawings BACKGROUND OF THE INVENTION 1. Field ofthe Invention The present invention relates to organic Solvent solu--tions containing acid-epoxy pressure-sensitive adhesive resins capableof self-cure for increased cohesivestrength, the self-curing reactionbeing catalyzed by a tertiary amine. In another aspect, the inventionrelates to a process of manufacture of articles containing films ofpressure-sensitive resin.

2. Description of the Prior Art Pressure-sensitive resins are usedconventionally in the form of films to provide adhesive bonds betweennormally non-adhering superstrates and substrates. The films may beprepared by casting from organic'solvent solutions and evaporating thesolvent.

Constraints are placed on the molecular weight of the pressuresensitivefilm may be subjected to stress gencrated by the weight of thesuperstrate or by dimensional change in the substrate orsuperstratecaused by thermal expansion, stress relaxation, plasticizer migra tionor the like. In general, a high molecular weight and a high cohesivestrength is desired in the pressuresensitive resin so that it may resistthe stresses induced in the adhesive film. The opposing requirements oflow molecular weight resin for ease of coating and high molecular weightresin for load holding ability are conventionally reconciled by the useof crosslinkable resin systems which can be applied at low molecularweight and cured to a high molecular weight.

Pressure-sensitive adhesive resins containing epoxy and acid groups arewidelyused because of their ability to self-crosslink on drying andheating to produce pressure-sensitive adhesive films with high cohesivestrength. However, the rate of curing of these systems is often too slowfor drying and curing conditions used in practice and a catalyst for theacid-epoxy reaction must then be added to the solution before coating.

Tertiary amines are widely accepted as catalysts for the acid-epoxyreaction. Generally, however, the curing rate with the majority oftertiary amines is still insufficient for the drying andcuringconditions used in practice. One tertiary amine which has provedto be ex-' ceptionally effective for the crosslinking reaction istriethylenediamine. Triethylenediamine has the disadvantage, however, ofconferring very short pot life to the pressure-sensitive resin solution.In commercial use,

pot life determines the useful coating life time of cata-" lyzed batchesof pressure-sensitive adhesive solutions. Catalyzed solutions with shortpot lives place severe limitations on the usefulness of the material,regardless of the properties of the adhesive. Furthermore,triethylenediamine causes an interaction between the pressure-sensitiveadhesive and silicone release substrates, particularly if the releasesubstrates arenot substantially completely cured. This leads toundesirably high adhesion between the adhesive and release surface withthe result that the release paper is difficult to remove and theadhesive properties of the pressure-sensitive adhesive film areimpaired.There is, therefore, a need in the art for acid-epoxy pressure-sensitiveresin: solu tions containing'a tertiary amine which possess adequate potlife which yield adequate rates of cure and which yield curedpressure-sensitive resins with satisfactory release properties fromrelease substrates.

SUMMARY OF THE INVENTION The above-mentionedneed in the art is fulfilledby the present invention of acid-epoxy pressure-sensitive resinsolutions containing a tertiary amine catalyst which have adequatepotlife. The pressure-sensitive solutions contain:

A. an organic solvent solution of an interpolymer comprising: v

1. between 0.1 and 15 weight per cent of an a,B -ethylenicallyunsaturated aliphatic carboxylic acid,

2. between 0.1 and 2 weight per cent of a glycidyl monomer selected fromthe group consisting of glycidyl acrylate, glycidyl methacrylate andallyl glycidyl ether,

3. between 35 to 84.9 weight per cent of a monomer selected fromthegroup consisting of esters of acrylic acid and methacrylic acidcontaining from 6 to 20 carbon atoms, and

4. optionally a monomer selected from the group consisting of a-olefinscontaining 2 to .10 carbon atoms, vinyl esters of alkanoic acidscontaining from 3 to 10 carbon atoms, ethyl and methyl esters of acrylicand methacrylic acids, acrylonitrile, methacrylonitrile, styrene, andvinyl chloride, wherein the interpolymerhas a weight average molecularweight in the range of 10,000 to 500,000 and a-glass transitiontemperaturein the range .of -l5 to ;C.; and

B. l ,3-bis(dimethylamino )-2-hydroxypropane; wherein there are between0.01,and l .0 parts by weight of the1,3-bis(dimethylamino)-2-hydroxypropane per parts by weight ofinterpolymer.

The invention-further provides articles of manufacture comprising-filmsof pressure-sensitive resin containing the abovedescribed interpolymerand 1 ,3-

bis(dimethylamino)-2-hydroxypropane and provides a processfor themanufacture of such-articles. 1

f I DESCRIPTION or T E INVENTION The practice of the present inventioninvolves the preparation of the interpolymer which is then formulatedwith the 1,3-bis(dimethylamino)-2- hydroxypropane. The solution is caston a substrate and the resulting film is dried and cured to ,apermanently tacky composition with improved cohesive strength.

The interpolymer comprises monomers selected from thegroups (1), (.2),(3) and (4) (described Group (2) monomers are selected from the groupconsisting of glycidyl acrylate, glycidyl methacrylate and allylglycidyl ether.

Group (3) monomers include the esters of acrylic and methacrylic acidcontaining from 6 to carbon atoms. Preferred esters contain branchedchain alkyl groups such as isobutyl acrylate, Z-ethylhexyl acrylate and2-ethylhexyl methacrylate. The pressure-sensitive resin need onlycontain monomers from Groups (1). (2), and (3). However, optionallymonomers from Group (4) may also be present. Group (4) includesa-olefins containing from 2 to 10 carbon atoms, vinyl esters of alkanoicacids containing 3 to 10 carbon atoms, such as vinyl acetate and vinyloctoate. ethyl and methyl esters of acrylic acid and methacrylic acids,acrylonitrile, methacrylonitrile, styrene and vinyl chloride.

The ratio of monomers in the interpolymer is selected so that the glasstransition temperature is in the range of-l5 to -75C. A suitable ratiois conventionally calculated from the equation:

where Tg is the glass transition temperature of the interpolymerexpressed as degrees Kelvin, Tg Tg etc. are the glass transitiontemperatures of the homopolymers of the respective comonomers and W Wetc. are the weight fractions of comonomers required for a specificglass transition temperature of the interpolymer. Glass transitiontemperatures may be determined experimentally by conventional methodssuch as by means of the duPont Differential Thermal Analyzer.

The weight average molecular weight of the interpolymer is in the rangeof 10,000 to 500,000, corresponding to a relative viscosity in the rangeof 1.1 to 7.0 measured on a 2 weight per cent solution of interpolymerin benzene. The preferred molecular weight range is from 20,000 to300,000, providing adequate cohesive strength to the interpolymerwithout excessive solution viscosity.

The interpolymers are conveniently prepared by the process set forth inU.S. Pat. No. 3,284,423 to Edmund C. Knapp.

The solids contents of the resin solutions of this invention shouldgenerally not exceed 50 percent by weight if the solution is to haveappropriate viscosity and pot life. In these respects, the preferredsolutions contain ethyl acetate-hexane in the ratio of between 5:] and3:2 and have a total solids content of about to per cent by weight and aviscosity within the range of about 100 to about 30,000 centipoises at25C. Furthermore, these resin solutions are compatible with many commonorganic solvents. For example, they may be diluted with equal volumes ofthe following solvents to give clear, useful solutions: acetone,isopropanol, butyl alcohol, 2-ethoxyethyl alcohol. 2- butoxyethylalcohol, carbon tetrachloride. ethyl acetate, hexane, methyl alcohol,methyl isobutyl ketone, toluene, 1,1,1-trichloroethane, and so on.

The method of polymerization is not critical in that the process can becarried out in solution or in emulsion. Care should be taken, however,to keep the reaction temperature below the level at which the acidcompound and the epoxide interact and cause gelation.

EXAMPLES l4 Resin Solution Compositions, Parts by Weight Ex. Ex. Ex. Ex.

Acrylic Acid 6.6 6.9 7.4 5.0 Glycidyl Mcthacrylate 0.07 0.2 0.35 AllylGlycidyl Ether 1.0 2-Ethylhexyl Acrylate 60.0 600 60.0 74.1) MethylAcrylatc 33.0 32.5 32.0 Vinyl Acetate 20 Ethyl Acetate 95.0 95.0 95.080.0 Hexane 19.0 190 19.0 Toluene 24.0

Formulation of the acid-epoxy pressure-sensitive resin solutions withthe amine catalyst is conveniently carried out by preparing a solutionof the amine in an organic solvent at a concentration between 5 and 50percent and adding the solution to the pressuresensitive resin solutionslowly with stirring to disperse the amine uniformly throughout. Theamount of 1,3- bis(dimethylamino)-2-hydroxypropane added to the resinsolution is between 0.01 and 1.0 parts by weight per parts by weight ofresin and is preferably between 0.05 and 0.4 parts by weight foradequate viscosity stability and curing rate.

The formulated solutions are subjected to determination of viscosityover a period of at least 24 hours at 25C. to determine the pot life.The viscosity is determined at 25C. by the conventional method using aBrookfield Model LVF Viscometer and the appropriate spindle and speed.

Data for Resin Examples 1 and 2 diluted to 30 per cent solids arepresented in Table 1. 1n the Tables which follow, triethylenediamine isdesignated as TDA and l,3-bis(dimethylamino)-2-hydroxypropane isdesignated as BDMAHP.

The data show that resin solutions containing 1.3-bis(dimethylamino)-2-hydroxypropane are vastly superior to resinsolutions containing triethylenediamine in viscosity stability and potlife.

The effect of catalyst on the curing rate is determined by comparing thecreep resistance of the catalyzed and uncatalyzed pressure-sensitiveresins and by comparing the degree of cure of the catalyzed anduncatalyzed resins.

CREEP RESISTANCE The pressure-sensitive resin solutions are cast onsilicone release paper and the cast films are dried for minutes at roomtemperature'and 2 minutes at 90F. The films (0.8 mil thicknessyarecooled to room temperature and transferred to a film of polyester soldunder the trademark MYLAR. The MYLAR film is cut into one-half inchstrips which are'applied to polished stainless steel to form 0.25 squareinch bonds. The bonds are held in a vertical plane at"70F.'and 50percent relative humidity and are loaded with one-pound weights. Thetime in hours for failure of the bond after application of the load isdetermined. Data for the resin of Example I cured for 2 minutes and for60'minutes at 100C. are presented in Table 2.

The data show that cured films containing 1,3-bis(dimethylamino)-2-hydroxypropane as the cure catalyst are superior incohesive strength to films of the same polymer without catalyst or withtriethylenediamine catalyst.

DEGREE OF CURE The degree of cure of the pressure-sensitive resin isdetermined by heating a dry film of the resin for 5 minutes at 110C. A 1gram sample of the resin is stirred in a lOO ml. graduated glasscylinder with 99 ml. toluene. The resin swells and dissolves at leastpartially. After 24 hours of stirring, the swollen resin is allowed tosettle in the graduate for 24 hours. The volume of swollen resin isnoted and the swelling index is calculated as the ratio of the volume ofswollen resin to the volume of the unswollen resin. A ml. sample of thesupernatant solvent is taken out through a plug of cotton into apipette. The resin content of the sample is determined, and, hence, thefractions of soluble resin and insoluble resin in the cured film aredetermined. in Table 3, data for resin Example 3 catalyzed withtriethylamine, triethylenediamine and l ,3-bis(dimethylamino)-2-hydroxypropane are compared with an uncatalyzedsample of resin. The tertiary amine catalyst is present in aconcentration of l-.78m.moles per 100 grams resin.

TABLE 3 The data show that triethylenediamine and 1.3-bis(dimethylamino) 2-hydroxypropane are very effective cure catalystscomparison with triethylamine. In gcneraLan insoluble fraction of 0.02or greater is asso- 5 ciated with appreciable cohesive strength in thepressure-sensitive resin.

KEIL RELEASE VALUES K eil release values for the pressure-sensitiveresins 0 are determined by coating a silicone release paper withpressure-sensitive resin solution and allowing the solution to dry for15 minutes. at 70F. and for 2 minutes at 90C. The dry film thickness is2 mil. MYLAR polycster film of 1 mil thickness is applied with thestandard 4-pound roller specified by the Pressure Sensitive Tape Councilto -the pressure-sensitive film. Strips of the MYLAR release paperlaminate 1 inch in width are cut. The strips are subjected to a T-peeltest on an lnstron Tensile Tester Machine, with a crosshead speed of 12inchesper minute. The peel value of Keil release value expressed asgrams per inch width is obtained. The release values, initial, after 24hours at 70C. and after 88 TABLE 4 Keil' Release Value. grams per inchwidth 24 Hrs. 70Cv 88 Hrs. Catalyst Initial 70C.

TDA BDMAHP Keil release values less than 30 grams per inch width aregenerally acceptable. The data show the rapid increase in Keil releasevalue of the resin containing triethylenediamine to an unacceptablevalue upon accelerated aging at 70.

The compositions of the present invention may be used as the adhesivecomponent in pressure sensitive tapes, films and foams. They adhere wellto resin surfaces such as plasticized poly(vinyl chloride) MYLAR,cellulose acetate, nylon, polyethylene and polypropylene, as well as topaper, metal and painted surfaces. They are especially useful as theadhesive component of decorative vinyl sheets and decals, conferringexcellent shrink resistance to vinyl film. Their outstanding tack,wetting and holding power may be used to advantage in transfer adhesiveapplications.

Articles of manufacture such as tapes, decals, decorative vinyl sheetsand transfer films containing the pressure-sensitive resin compositionof the present invention are prepared by coating the resin on theappropriate substrate by conventional coating methods. Such articlesconventionally include a release paper for temporary protection of theadhesive film until the adhesive bond is made. The thickness of theadhesive film is generally in the range of 0.2 to 5 mil.

Application of the film to the substrate is conventionally carried outon roll coaters such as reverse roll and gravure roll coaters. The resinsolution viscosity is adjusted to between 25 and 5,000 centipoises withhigher viscosities within the range preferred for reverse roll coatingand lower viscosities within the range preferred for gravure coating.The coatings are applied at a rate of between 10 feet per minute andL000 feet per min ute. I a

While the present invention has been described with particular referenceto certain specific embodiments thereof, it will be understood thatcertain changes, substitutions and modifications may be made thereinwithout departing from the scope thereof. This invention alsocontemplates the use of fillers, extenders, stabilizcrs, antioxidants,plasticizers tackifiers, flow control agents, adhesion promoters, dyes,etc. in the pressuresensitive resin solutions and the pressure-sensitiveresins of this invention.

What is claimed is:

1. A process of manufacture of a pressure-sensitive adhesive coatedarticle which is readily released from silicone release surfaces andwhich provides bonds of high cohesive'strength, comprising: i

A. polymerizing in an organic solvent medium an interpolymer containing:

1'. between 0.] and weight per cent of an a,,B-ethylenically unsaturatedaliphatic carboxylic acid,

2. between 0.l and 2 weight per cent of a glycidyl monomer selected fromthe group consisting of glycidyl acrylate, glycidyl methacrylate andallyl glycidyl ether,

3. between 35 to 84.9 weight per cent of a monomer selected from thegroup consisting of esters of acrylic acid and methacrylic acidcontaining from 6 to carbon atoms, and

4. optionally, a monomer selected from the group consisting of a-olefinscontaining from 2 to l0 carbon atoms, vinyl esters of alkanoic acidscontaining from 3 to 10 carbon atoms, ethyl and methyl esters of acrylicand methacrylic acids, acrylonitrile, methacrylonitrile, styrene andvinyl chloride, wherein the interpolymer has a weight average molecularweight in the range of l0,000 to 500,000 and a glass transitiontemperature in the range of l 5 to -C.; and

B. adding between 0.01 and 1.0 partsv ofvthe l,3- 7

bis( dimethylamino )-2-hydroxypropane I00 parts of interpolymer; and C.coating the interpolymer solution on the article. 2. The process ofclaim 1 wherein the a, B-ethylenically unsaturated aliphatic carboxylicacid is selected from the group consisting of acrylic acid, methacrylicper acid, crotonic acid, isocrotonic acid, maleic acid, fu-

4. The process of claim 1 wherein the glycidyl monomer is glycidylmethacrylate.

5. The process of claim 1 wherein the article is silicone coated releasepaper.

1. A PROCESS OF MANUFACTURE OF A PRESSURE-SENSITIVE ADHESIVE COATEDARTICLE WHICH IS READILY RELEASED FROM SILICONE RELEASE SURFACES ANDWHICH PROVIDES BONDS OF HIGH COHESIVE STRENGTH, COMPRISING:A.POLYMERIZING IN AN ORGANIC SOLVENT MEDIUM AN INTERPOLYMERCONTAINING:
 1. BETWEEN 0.1 AND 15 WEIGHT PER CENT OF AN A,B-ETHYLEICALLYUNSATURATED ALIPHATIC CARBOXYLIC ACID,
 2. BETWEEN 0.1 AND 2 WEIGHT PERCENT OF A GLYCIDYL MONOMER SELECTED FROM THE GROUP CONSISTING OFGLYCIDYL ACRYLATE, GLYCIDYL METHACRYLATE AND ALLYL GLYCIDYL ETHER, 2.The process of claim 1 wherein the Alpha , Beta -ethylenicallyunsaturated aliphatic carboxylic acid is selected from the groupconsisting of acrylic acid, methacrylic acid, crotonic acid, isocrotonicacid, maleic acid, fumaric acid, citraconic acid, itaconic acid, and thealkyl monoesters of maleic acid, fumaric acid, citraconic acid anditaconic acid in which the alkyl group contains from 1 to 4 carbonatoms.
 2. between 0.1 and 2 weight per cent of a glycidyl monomerselected from the group consisting of glycidyl acrylate, glycidylmethacrylate and allyl glycidyl ether,
 3. between 35 to 84.9 weight percent of a monomer selected from the group consisting of esters ofacrylic acid and methacrylic acid containing from 6 to 20 carbon atoms,and
 3. The process of claim 1 wherein the Alpha , Beta -ethylenicallyunsaturated aliphatic carboxylic acid is acrylic acid or methacrylicacid.
 3. BETWEEN 35 TO 84.9 WEIGHT PER CENT OF A MONOMER SELECTED FROMTHE GROUP CONSISTING OF ESTERS OF ACRYLIC ACID AND METHACRYLIC ACIDCONTAINING FROM 6 TO 20 CARBON ATOMS, AND
 4. OPTIONALLY, A MONOMERSELECTED FROM THE GROUP CONSISTING OF A-OLEFINS CONTAINING FROM 2 TO 10CARBON TOMS, VINYL ESTERS OF ALKANOIC ACIDS CONTAINING FROM 3 TO 10CARBON ATOMS, ETHYL AND MTHYL ESTERS OF ACRYLIC AND METHACRYLIC ACIDS,ACRYLONITRILE, METHACRYLONITRILE, STYRENE AND VINYL CHLORIDE, WHEREINTHE INTERPOLYMER HAS A WEIGHT AVERAGE MOLECULAR WEIGHT IN THE RANGE OF10.000 TO 500,000 AND A GLASS TRANSITION TEMPERATURE IN THE RANGE OF-15* TO -75*C., AND B. ADDING BETWEEN 0.01 AND 1.0 PARTS OF THE1,3-BIS(DIMETHYLAMINO)-2-HYDROXYPROPANE PER 100 PARTS OF INTERPOLYMER,AND C. COATING THE INTERPOLYMER SOLUTION ON THE ARTICLE.
 4. The processof claim 1 wherein the glycidyl monomer is glycidyl methacrylate. 4.optionally, a monomer selected from the group consisting of Alpha-olefins containing from 2 to 10 carbon atoms, vinyl esters of alkanoicacids containing from 3 to 10 carbon atoms, ethyl and methyl esters ofacrylic and methacrylic acids, acrylonitrile, methacrylonitrile, styreneand vinyl chloride, wherein the interpolymer has a weight averagemolecular weight in the range of 10,000 to 500,000 and a glasstransition temperature in the range of -15* to -75*C.; and B. addingbetween 0.01 and 1.0 parts of the1,3-bis(dimethylamino)-2-hydroxypropane per 100 parts of interpolymer;and C. coating the interpolymer solution on the article.
 5. The processof claim 1 wherein the article is silicone coated release paper.